A steerable drilling system is used to drill a deviated borehole from a straight section of a wellbore. Steerable drilling systems conventionally use a downhole motor (mud motor) powered by drilling fluid pumped from the surface to rotate the drill bit. Most commonly, a positive displacement motor of the Moineau type, which uses a spiraling rotor that is driven by fluid pressure passing between the rotor and stator, is employed. Such mud motors are capable of producing high torque, low speed drilling that is generally desirable for steerable applications.
In an example implementation, the motor and bit are supported from a drill string that extends to the well surface. The motor is operable to rotate the bit via a constant velocity (CV) drive linkage that extends through a bent sub or bent housing positioned between the power section of the motor and a bearing assembly of the motor. In addition to accommodating power transmission over the bend angle, the CV linkage allows for the spiraling nutation of the power section of the mud motor.
Bent housings (fixed or adjustable) are used as part of the mud motor to alter the direction of the drill bit drilling a wellbore. Usually the bent housing will move the tool face, i.e., the face of the drill bit that is engaging the formation, from 1 to 5 degrees off of the centerline of the drill string and wellbore, thereby causing a change in the direction of the wellbore.
Rotary drilling, wherein the drill string is rotated from the rig at the surface, is used to drill the straight sections of the borehole. The mud motor and bent sub are rotated with the drill string, resulting is a slightly enlarged borehole to be drilled. To steer the bit, however, the operator holds the drill string from rotation and powers the downhole motor to rotate the bit. The non-rotating drill string and mud motor assembly slide forward along the borehole during penetration. During this sliding operation, the bend directs the bit away from the axis of the borehole to provide a slightly curved borehole section, with the curve achieving the desired deviation or build angle.
Mud motors generally consists of a bent housing whose bend angle cannot be controlled while downhole. In order to change the inclination of the bent housing, it is necessary to pull the bent housing from the borehole (called “tripping out”) to change the inclination setting. Tripping out of borehole increases nonproductive time. It is desirable to have a system or a mechanism that allows the operator to change the inclination of the bent housing while downhole.